Page:Popular Science Monthly Volume 63.djvu/229

Rh not learn from them the nature of that process, but one of its limits. Moreover, the probability that these laws are of general application is greatly lessened by the fact that they are demonstrable only in connection with narrowly inbred and much divergent varieties of plants and animals, to which condition of the experiment the phenomena discovered may prove to be due, rather than to any general fact or mechanism of heredity. It seems certain, however, that neither theory nor experiment will make permanent progress in this direction as long as we continue to confuse under the word hybrid several extremely diverse evolutionary conditions, and fail to realize that generalizations based on any one kind or type of hybrids are quite premature and irrational.

Sterile Hybrids.—The original notion of a hybrid, or at least the most popular meaning of the term, is that of a cross between organic types so widely diverse that the progeny are in some way abnormal or defective, especially with reference to reproduction. Among animals sterile hybrids can not be propagated, but in plants they can be grown from cuttings or buds, and are thus preserved as horticultural * varieties. '

Aberrant Hybrids.—The second and succeeding generations of hybrids not completely sterile often show striking deviations from both parental types. As these new characters are analogous to the abrupt variations of close-bred plants described by Darwin as 'sports' and more recently renamed 'mutations' by De Vries, it has been suggested that they may be due to the same causes, that is, they may not be in reality the result of crossing, but rather of an inadequate conjugation or defective fertilization which allows a lapse from the normal form. Both mutations and mutative hybrids are comparatively infertile, so that their suddenly attained new characters should not be looked upon as true contributions to evolutionary progress.

Reciprocal or Mendelian Hybrids.—Mendel and his successors have proved that there is still a third type of less abnormal hybrids, in which there is no permanent combination or averaging of divergent parental characters, although it is not known that vigor and fertility are notably diminished. Mendel's so-called laws are generalized statements of the results of his experiments upon the crossing of different garden varieties of the pea; he himself found that the same was not true among hybrids of Hieracium. A part of the scientific